1. Field of the Invention
The present invention relates in general to a linear index and, more particularly, to a linear index for dividing a total driving section of a rectilinear driving device, comprising a pneumatic cylinder, a hydraulic cylinder or an electric motor with a lead screw, at regular intervals and for forcibly driving and stopping the device step by step using electric logic signals.
2. Description of the Prior Art
As well known to those skilled in the art, a low-priced pneumatic cylinder, a kind of representative rectilinear driving device, has a simple construction and is recently necessarily used in automation of varieties of industrial machinery.
With reference to FIG. 1, there is shown a typical pneumatic cylinder assembly as a rectilinear driving device. This typical pneumatic cylinder assembly comprises a programmable logic controller (PLC) 1, a solenoid valve 2 and a pneumatic cylinder 3. A carrier 5 is coupled to the free end of a piston of the cylinder 3 and is rectilinearly moved along a linear bearing 4 in accordance with advancing and refraction of the piston. In this pneumatic cylinder assembly, the carrier 5 coupled to the piston of the cylinder 3 is rectilinearly moved between opposed ends of a stroke section D when a drive control signal of the PLC 1 is applied to the solenoid valve 2.
When it is required to stop the piston of the cylinder 3 at a random position of the stroke section D of the pneumatic cylinder assembly, a pair of proximity sensors 6a and 6b are placed above the moving path of the carrier 5 as shown in FIG. 2. Each of those sensors 6a and 6b senses approach of the moving carrier 5 and outputs a sensing signal to the PLC 1. Upon reception of the output signals of the sensors 6a and 6b, the PLC 1 outputs a logic signal to the solenoid valve 2, thus to automatically close this valve 2 and to stop the piston of the cylinder 3 at the desired random position of the stroke section D.
However, since the air for driving the piston of the above pneumatic cylinder 3 has a high compressibility and the piston of the cylinder 3 operates with a relatively higher frictional force, the probability in that the practical stop position of the piston is within the allowance of the designated stop position is statistically very low even though the solenoid valve 2 is correctly closed at the designated position.
One approach to overcoming the above problem is use of pneumatic piston brake means about the piston rod of the cylinder 3 as shown in FIG. 3. The pneumatic piston brake means comprises a pneumatic brake 8 driven by a solenoid valve 7 and improves the precision of stopping of the piston at the stop position. However, this technique using the pneumatic piston brake means has a problem in that it can not assure the piston of high precision of then stop position when the response velocity of the piston brake means is not uniform.
In a rectinlinear driving device using proximity sensors, one should increase the number of the proximity sensors proportion to the number of desired stop positions of piston in the stroke section D. Therefore, this type of rectilinear driving device has another problem in that it should require a complex logic controller which is adapted to compare the present position of the piston with a designated position using the output signals of the proximity sensors 6a and 6b and to apply a logic signal, corresponding to the logic condition, to the solenoid valve 2.
Another problem of the above rectilinear driving device resides in that its proximity sensors should be displaced when the stop positions of the piston are changed.
In this regard, it is noted that, a rectilinear driving device comprising a lead screw combined with either of a step motor and a position controllable servo motor is preferably used in a rectilinear driving operation in which a precise stop function for stopping the piston of the cylinder at several stop positions should be required together with random change of the stop positions.
Referring to FIG. 4, there is shown a typical rectilinear driving device having a step motor and a lead screw. In operation of this rectilinear driving device, the electric step motor 11 is driven when a drive signal of a stepping motor drive unit 9, which drive unit 9 has been programmed with a driving direction and a step number, is applied to the step motor 11 coupled to reduction gears 10. The rotational force of the step motor 11 is transmitted to the lead screw 12 coupled thereto and rotates this lead screw 12. This lead screw 12 is rotatably supported at its opposed ends by a pair of radial bearings 13a and 13b. In accordance with rotation of the lead screw 12, a carrier 14 coupled to this lead screw 12 is rectilinearly moved under the guide of a linear bearing 15 or stopped at a predetermined stop position.
However, this type of rectilinear driving device having a step motor or a positional controllable servo motor has a problem in that it increases the cost due to the motor.